The MOV is subjected to a baseline voltage measurement with 100mA across the terminals. Then a 8x20 mS impulse current waveform with a peak of x kA (perhaps 2,500A) is applied. Then the voltage is remeasured with 100mA across the terminals. If it is still within +/- 10%, that kA rating is OK. The current is then boosted (say x+100A) remeasured, tested, remeasured, ... When the 100mA measurement falls outside the +/- 10% allowable window, the last kA measurement was valid.

Some mfrs state kA ratings with 1 and 2 pulses. You will see a lower kA rating with the 2 impulse because the part has already been hammered once , it is warmed up, the material within the part has not completely 'reset' itself , and
the second impulse must be within a specified time interval. The longer you can wait, the cooler the part, the better it can handle the next impulse.

In conclusion, it's the maximum peak current, using an 8 x 20mS waveform, that a MOV can handle where the 100mA voltage measurement test deviates less-than +/- 10% from the original baseline."